It is known that for magnetic or paramagnetic particles the rate of precipitation and the adhesion of the particles to the fibres of the filter can be increased by a so-called high gradient magnetic filter. In such a filter a matrix of ferromagnetic fibres are located in a strong magnetic field. High magnetic gradients adjacent the fibres magnetically attract the particles to the fibres with a force greater than that due to diffusion or inertia. Further, the magnetic force holding the particles to the fibres is considerably greater than the normal van der Waals forces.
As an example, a high gradient magnetic filter can comprise a ferromagnetic wire wool matrix which is contained in a stainless steel can, the can being located between the poles of a magnet. The filter can be cleaned by removing the magnetic field and flushing with a liquid to remove captured particles.
It is found in practice that with a wire wool matrix a large quantity of liquid is required to clean the matrix. In addition, the wire matrix filler has a small volume filling or packing factor, defined as the ratio of the volume of the wire wool to the total volume of the can, and as a result the quantity of particles the filter can hold is limited. Also, as the efficiency of the filter decreases as soon as particles are captured thereon, it suffers the disadvantage that it must be cleaned at relatively frequent intervals. This is particularly relevant when the wire matrix is used to filter a continuous flow therethrough.
Accordingly, the present invention seeks to provide an improved form of magnetic filtration which does not rely on the above mentioned wire wool matrix filter.